1. Field of the Invention
This invention relates generally to electronic circuits designed to exercise a control function by turning on small motors, heaters, water valves, etc. related to car washing machinery, laundry machinery, fluid dispensers or any other unattended equipment which can be controlled by deposit of coins for sale of a service of predetermined duration.
2. Description of the Prior Art
In the past and to a great extent at present, appliance controls requiring timed on-off cycles incorporate composite electromechanical switches consisting usually of a time-delay secondary relay controlling a primary contactor which accomplishes the power switching function. Design of the secondary relay depends on time interval considerations. Orificed pneumatic or hydraulic dash-pot arrangements are used for delays of 30 seconds to several minutes while switch contactors driven by synchronous clock motors can be used for "on" or "off" cycles of 30 seconds to weeks or even months. Bimetallic thermal switches with fixed or variable time intervals based on thermal inertia have also been successfully used for secondary controls.
As highly reliable and comparatively low-priced semiconductor products became available, it became practical to construct all electronic counterparts of many types of electromechanical controllers. Such electronic counterparts are in most cases cheaper and more reliable. These solid state controls also show promise of long, maintenance-free lifetimes and high resistance to degrading environmental factors such as temperature, humidity and exposure to dirt and grease. These features are important considerations with regard to controls for laundry and related machinery or other equipment operating under similar environmental conditions.
Accordingly, a need is perceived to exist for an electronic circuit which can be incorporated into a hermetically sealed container with a 550 watt "on" cycle subject to a nearly unlimited choice of time intervals and with the "on" cycle initiated by momentary closure of an external circuit with less than 10 volts of lead-to-lead potential and with less than one-half milliampere of current at the moment of circuit closure. It is believed that an electronic switching module with these characteristics will have a variety of industrial control applications.